Internally, the potentiometer has a resitive substrate with connections on both ends, pins 1 and 3. It also has a wiper that can be moved along the substrate, pin 2.

The audio signal flows out of the preamp circuit to the potentiometer. The resistance across the potentiometer is the load to the preamp circuit. One side of the pot is connected to the preamp output (input to the pot) and the other side is connected to ground.

The wiper rests on some point between the signal side and ground. This makes two resistances from this point, one to ground and another to the preamp signal. This forms a voltage divider, and the signal level depends on the position of the wiper.

To wire this up properly, you will need to know what wires go to what points in the circuit. You need to identify why lines are ground, what lines are from the preamp output and what lines are inputs to the next stage.